1. Field of the Invention
This invention relates to light emitting devices and more especially to packaging arrangements for such devices. In particular, although not exclusively, the invention relates to light emitting devices which include one or more phosphor (photo-luminescent) materials to generate a desired emission product color. Moreover embodiments of the invention concern, but are not limited to, white light generating devices for general lighting applications with a high emission intensity (i.e. a luminous flux of more than 500 lumens) that are based on light emitting diodes (LEDs).
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip generates blue light and the phosphor material(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor material provides light which appears to the human eye as being nearly white in color.
Currently there is a lot of interest in using high brightness white LEDs as energy efficient replacements for conventional incandescent light bulbs, halogen reflector lamps and fluorescent lamps. Most lighting devices utilizing LEDs comprise arrangements in which a plurality of LEDs replaces the conventional light source component. A problem with such existing light emitting devices, in particular white light emitting devices intended for general lighting which typically require an emitted luminous flux of 500 lumens or greater (i.e. an input power of typically 5 W to 10 W), is thermal management and in particular adequately dissipating heat generated by such devices. A further problem is thermal degradation of the phosphor material with time which can result in a significant change in the correlated color temperature (CCT) and/or luminous flux of light emitted by the device. Typically the packaged device is mounted on a heat sink or other large thermal mass to dissipate heat. However, the thermal resistance of the package can make it difficult to adequately transfer heat from the LED chips (dies) through the package which is often fabricated from an electrically insulating material such as a high temperature polymer or ceramic.
An example of a white LED package is disclosed in co-pending U.S. patent application Publication No. US 2009/294780A1 (Published Dec. 3, 2009) to Chou et al. A schematic sectional view of such a package is shown in FIG. 1. The white light emitting device 10 comprises a square array of blue (i.e. wavelength≈400 to ≈480 nm) surface emitting InGaN/GaN (indium gallium nitride/gallium nitride) based light emitting diode (LED) chips 12 packaged in a high temperature package 14 such as for example a low temperature co-fired ceramic (LTCC) package. The package 14 has in an upper surface a square array of circular recesses (cups) 16, each of which is configured to receive a respective LED chip 12. The package 14 further comprises a pattern of electrically conducting tracks 18 that define electrical contact pads on the floor of each recess 16 and a thermally conducting mounting pad 20. The mounting pad 20 is thermally connected with a corresponding mounting pad 22 on the base of the package by an array of thermally conducting vias 24. The package 14 further comprises electrical connectors 26, 28 on the base of the package for providing electrical power to the device. Each LED chip 12 is mounted to the mounting pad 20 of a respective recess 16 using for example a thermally conductive adhesive or eutectic soldering. Electrode contacts on the upper surface of the LED chip 12 are electrically connected to a corresponding electrical contact pad on the floor of the recess by bond wires 30 and each recess 16 is completely filled with a transparent polymer material 32, typically a silicone, which is loaded with a powdered phosphor material 34 to ensure that the exposed surfaces of the LED chip 12 are covered by the phosphor/polymer material mixture. To enhance the emission intensity (luminous flux) of the device the walls of the recess are inclined such that each recess comprises a reflector. Whilst such a device package performs well both electrically and thermally (i.e. thermal resistance is typically 8 to 10° C./W), such packages are relatively expensive to fabricate which can limit the applications for which they are cost effective.
Another packaging arrangement comprises mounting the LED chip directly on a metal core printed circuit board (MCPCB) and then encapsulating the LED chip with a phosphor encapsulation. Such a configuration is often termed a chip on board (COB) arrangement. As is known an MCPCB comprises a layered structure comprising a thermally conducting base, typically aluminum, an electrically non-conducting/thermally conducting dielectric layer and electrically conducting circuit layer typically made of copper. The dielectric layer is very thin such that they can conduct heat from components mounted on the circuit layer to the base. As disclosed in co-pending U.S. patent application Ser. No. 12/689,449 entitled “Light emitting devices with phosphor wavelength conversion and methods of manufacture thereof” (filed Jan. 19, 2010) to Li et al. the phosphor encapsulation can be in the form of a substantially conformal coating that is molded on the LED chip. Alternatively, as disclosed in co-pending U.S. patent application Ser. No. 12/639,688 entitled “Light emitting devices with phosphor wavelength conversion” (filed Dec. 16, 2009) to Li et al. the phosphor material can be provided on the inner surface of a light transmissive hemispherical shell that is mounted over the LED chip to form a moisture/gas tight enclosure. It is also known to incorporate the phosphor material within a hemispherical lens that is formed over the LED chip by for example molding. Whilst the thermal resistance of an MCPCB is excellent and can be as low as 1.5° C./W the cost of such a packaging arrangements, in particular the cost of applying phosphor material to individual LED chips, can be prohibitively expensive in the highly competitive cost conscious general lighting sector. Moreover with such a packaging arrangement it can be difficult to produce devices with a consistent CCT.
It has also been proposed to mount an array of LED chips on an MCPCB and then encapsulate the entire array of the LED chips with a phosphor coating that is contained within a peripheral frame (for example a circular annular frame) mounted to the MCPCB such as to surround the array of LEDs. Whilst a package with a single recess is easy to fabricate such an arrangement requires additional phosphor material to fill the regions between adjacent chips and that such phosphor material contributes little or nothing to light generation for a surface emitting LED chips. It is believed that the additional phosphor material can absorb light and reduce the devices overall light emission. Initial tests suggest that a package having a single recess that houses a number of LED chips has a lower luminous efficacy (e.g. 50 to 55 lm/W) compared with a package in which the same number of LED chips are each housed in a respective cavity (e.g. 70 to 80 lm/W). Moreover, it is believed that thermal degradation of the phosphor material maybe higher in a device where a single recess housing multiple LED chips.
A need still exists for a less expensive light emitting device package with a thermal resistance that is comparable with an MCPCB and which enables easy application of the one or more phosphor materials.